While Japan does have a pacifist constitution there is no telling how it would react to attacks on its soil especially nuclear ones even if only targeted at US bases.The death toll would still be extremely high.So the russians would be wise to take out japanese bases as well at least all bases on Hokkaido which is the closest to russian soil and Tokyo to take out the japanese government.We must add tensions between the two linked to the Kuril Islands dispute.This has never been solved even today.Russian thinking might be in a post-war world surviving japanese forces would be likely to consider this japanese soil and attack.
Even today Article 9 of the Japanese constitution specifically forbids agressive military action overseas and even close to home unless it is purely for self-defense purposes (e.g. intercepting and shooting down inbound Russian bombers once they are actually in Japanese air space). It precludes any retalitory action on the part of Japanese forces, so even if they were attacked (by Russia, China, North Korea or anyone for that matter), the country's constitution does not allow for any strike back.
So even though Japan has tried to 'help' allies in Afghanistan and elsewhere, it's been limited to purely helping out behind the front lines, offering aid and medical facilities and even then, this has been hugely controversial in Japan.
There has been a lot of noise in recent years about repealing Article 9 to allow Japan to assert itself around the world, effectively looking to punch at its (still considerable) economic weight. This again is controversial at home and overseas, the Chinese and Koreans are v
ehemently against this and is stifiling some of the debate. Why? Japan has a kind of national pre-occupation of what the outside world thinks of Japan (even China and Korea). Tabloid ('sport') newspapers often carry articles on this, daily 'wide shows' on TV often debate what the foreign media is saying about Japan. This was the same in the 1980s as it is now.
In the 1980s Article 9 was still entrenched and debated perhaps less than it is now. Whole generations of Japanese schoolkids were schooled in the thought that war is wrong, war is evil and that they should forever be at peace. This was kind of forced into Japanese education by the Americans and to be honest, this 'indoctrination' of Japanese schoolkids is pretty brutal, even now. Japanese educators took to educating pacifism just as earnestly as they had indoctrinated kids into believing that the Emperor was God and that Japan had a divine role in creating its empire. So elementary school kids have to read (and watch the anime) 「はだしのゲン」('Barefoot Gen') which is very harrowing and even disturbed me when I read the manga for the first time aged 18. They have to learn about the firebombing of Tokyo through 「火垂るの墓」 ('Grave of the Fireflies') and are conditioned, through being pretty much scared into not being able to sleep at night (they read / view these things at around the age of 8 / 9), into believing that war is wrong and that they should follow a pacifist lifestyle.
If you've never read / seen these, there are English language versions available. They're pretty harrowing and kind of help to show how the Japanese learn from an early age, even now, about war. I can recommend them.
So it's not really so much that the constitution is pacifist (it is), it's also the case that the general population is 'indoctrinated' (maybe there's a better word than than?) into being pacifist.
Whether or not its a realistic assumption is beyond the point.Plus even today exactly what measures are in place for a defence of Japan by the US are classified so russian strategy has to assume the most likely course of action regardless of official statements.So the japanese are pretty badly hit regardless Hokkaido is most likely wiped off the map at the very least alongside US bases,japanese air and navy bases,and some civilian airport and port facilities and some nuclear power plants just to be sure.Japanese army installations although in Japan the term isn't used ground-self defence being official are unlikely to be hit except for those on Hokkaido.Without an air and naval capability the army in the rest of the country is pretty much impotent to mount anything like an attack.
Yup, that's true - even the Japanese know very little of the US plans!
Here's the thing though, Japan would not react if US forces were targeted, even in Japan! It would be seen as an attack on US sovereign bases and would not be dealt with by Japan because of Article 9 and the treaties that are in place between Japan and the US.
There's a really good overview of this alliance published US Congress's Congress Research Service all about this kind of thing - you can read the PDF here:
http://www.fas.org/sgp/crs/row/RL33740.pdf
This would have been well known to Russian strategists in 1983 and 1984 and they would know that the Japanese would not strike back. Even if they wanted to, the Japanese politicians would be too busy bickering about repealing Article 9, the general public would be more-or-less against any offensive strikes that the government would be evaporated into the air as the ICBMs slam into Kasumigaseki and other places in Tokyo and the greater Kanto region.
However, these are valid points you have raised - would Article 9 have been repealed? Would Japan have tried to make its own nukes ahead of the exchange? Would the public be pro-revenge on whoever nuked them (if the attacker could be proven) given that getting revenge on people, no matter how long it takes, is something that runs deep in Japanese society? These are difficult questions to answer but the most obvious answer would be 'no' to each of them. But that may not be the case and is something that I'll be looking at through the development of the P&S Asia Pacific thread.